The present invention relates to a differential cylinder having a cylinder and a piston and operable to move the piston under hydraulic forces.
There are known hydaulic differential cylinders, used for servo control or the like, which comprise a cylinder, a piston slidably fitted in a cylinder chamber defined in the cylinder, and a piston rod having one end coupled to the piston and the other end projecting out of the cylinder, the piston and the piston rod being movable by supplying hydraulic pressure into the cylinder chamber divided by the piston.
For example, such a differential cylinder may be employed as a servo cylinder for controlling clutch operation and speed reduction ratio of a hydraulically operated continuously variable speed transmission.
The proposed differential cylinder comprises, an shown in FIG. 5 of the accompanying drawings, a cylinder 51, a piston 52 slidably fitted in a cylinder chamber defined in the cylinder 51, and a piston rod 53 coupled to the piston 52 and projecting out of the cylinder 51. The cylinder chamber is divided by the piston 52 into a righthand rod-side cylinder chamber 55 and a lefthand head-side cylinder chamber 54. The rod-side cylinder chamber 55 is supplied with working fluid under constant hydraulic pressure Pc from a hydraulic pressure supply source 56 through a hydraulic line 61. The head-side cylinder chamber 54 is supplied with working fluid through a hydraulic line 62 under control hydraulic pressure Ps which is produced by reducing the hydraulic pressure Pc from the hydraulic pressure supply source 56 with two duty-ratio controlling solenoid valves 57, 58. The head-side cylinder chamber 54 has a piston-pressure-bearing area Ah of the rod-side cylinder chamber 55. Therefore, the piston 52 and the piston rod 53 can be moved horizontally by varying the control hydraulic pressure Ps acting in the head-side cylinder chamber 54 in a pressure range which is lower than the constant hydraulic pressure Pc.
With the differential cylinder of the above structure, the maximum value of a force Fl for pushing the piston 52 to the left when the control hydraulic pressure Ps is zero is given by: EQU Flmax=.pi./4(A.sup.2 -B.sup.2) *Pc
(where an asterisk, or * mark, represents a multiplication)
whereas the maximum value of a force Fr for pushing the piston 52 to the right is produced when the control hydraulic pressure Ps is equal to the constant hydraulic pressure Pc, and is given by: EQU Frmax=.pi./4* B.sup.2 * Pc.
Therefore, in order to increase the rightward force Frmax to push the piston 52, the diameter B of the piston rod 53 has to be increased. If the diameter B is increased, however, the leftward force Flmax for pushing the piston is reduced. Consequently, the above differential cylinder cannot produce large forces to push the piston 52.
Various efforts have been made in the past to increase the piston pushing forces by increasing the cylinder diameter, or by employing a four-way valve to vary the hydraulic pressure supplied to the rod-side cylinder chamber 55. However, these proposals result in a large-size differential cylinder or a complex control system.